The present invention concerns the rims for tires, a complete rim-wheel assembly and, in particular, it concerns the rims provided with devices to prevent tire bead unseating.
When a vehicle travels in a curve, the sidewall of the vehicle tires are subjected to a force, directed substantially parallel to the axis of the wheel itself, the intensity of which varies depending upon the speed of the vehicle and the radius of curvature of the path.
This force acts also on the bead of the tire (in a direction towards the axially inner zone of the rim), and it is counteracted almost exclusively by the inflation pressure of the tire, so much so that, in the instance of even a partial deflation of the tire, when this counteracting action is reduced, it could happen that the tire bead may shift from its normal seat on the rim and drops into the central well which is normally used for tire fitment. As a consequence of this, almost certainly there results a loss of driving control by the driver over the vehicle, involving dangerous risks and sometimes even tragic consequences to life itself.
For this reason, the problem of preventing any shifting of the bead has always been considered important, but it has now become still more important with increase of vehicle speed, which has become more likely because of improvements in road conditions and improved driving performance of vehicles themselves.
To this purpose, for some time past, there has been planned and there are, moreover, already known, many devices intended to maintain the bead on the rim. Among these devices, those that are more generally adopted consist of a particular protrusion better known as a "hump", which is formed on the rim in the vicinity of the axially inner extremity of the bead seat, protruding radially outwardly for a constant height with respect to the bead seat, and which is intended to constitute an insurmountable obstacle in the axial shifting path of the bead.
A practical consideration is the necessity for the tire's carcass bead to be able to pass over said "hump" during the mounting of the tire upon the rim. The height of said "hump", with respect to the bead seat, cannot exceed a certain given value. In fact, if the height of said "hump" is so raised as to positively block the bead on the tire rim, then it will be impossible to mount the tire on a one-piece rim.
The height of said "hump" thus must be of such a value that it is possible for the tire bead to pass over said "hump" (during the mounting phase of the tire onto the rim), by exploiting the flexibility of the metallic bead core, which enables the bead core, although inextensible in circumference, to assume an elliptical configuration, and by exploiting the compressibility of the elastomeric material that covers the radially inner surface of said metallic bead core.
Thus, there is obtained a characteristic compromise between having a certain degree of difficulty in mounting the tire, but also having a corresponding sealing capacity of the rim to the bead when the vehicle is cornering with a partially deflated tire, down to about 60% of the normal inflation. To improve this situation, there have been proposed rims having asymmetrical "humps", i.e., with a variable height along at least one part of the rim circumference, from a minimum value to a maximum value and vice-versa.
In reality, the above-mentioned variant has not completely solved the problem. In fact, if, in spite of the increase in height of the "hump" along a portion of the circumference of the bead seat the circumferential development of said "hump" is maintained constant, the ease of mounting of the tire onto the rim will not be improved. While, with regard to the bead unseating ability, the behavior of the rim will become unsatisfactory along the portion of the rim circumference where said height is less as compared to the normal heights of the usual "humps".
On the contrary, if a greater height is obtained by increasing the height of a usual symmetrical "hump" along a portion of its development, in such a way that the total hump circumference increases, it will surely result in less bead unseating; but, at the same time, also a notable increase in the difficulty in mounting the tire on the rim.
There has been proposed by the applicants themselves, in copending application Ser. No. 4,80,329 now U.S. Pat. No. 4,502,521, filed Mar. 30, 1983, rims having two or more "humps" placed side-by-side, each one having a circumference such as to be able to easily be crossed-over by the tire bead during the mounting of the tire onto the rim, but which, owing to the fact that such "humps" are eccentrically disposed with respect to the rim axis taken as a whole, constitute an obstacle having a considerable height for the tire bead, resulting in a sure advantage with regard to resisting any bead unseating.
Nevertheless, these side-by-side "humps" are not completely satisfactory, owing to the fact that during the tire-mounting phase they allow leakages (although slight) of air, until such time as the beads are stably housed on their seats. Moreover, under particular conditions of use, each "hump" can represent a fulcrum point for the bead to overcome; thus inducing risky and dangerous bead rotation around said fulcrum.